The phylum Chaetognatha is a mysterious group of organisms that has eluded
scientists for more than a century because of their unique morphology and
developmental characteristics, i.e. protostome (mouth develops from blastopore; e.g.
mollusks, annelids, arthropods) versus deuterostome (anus develops from blastopore;
e.g. echinoderms and chordates) offer few clues to their evolutionary origins. Some early
morphological studies argued that chaetognaths were derived mollusks or nematodes
according to gross ultrastructural data, while other studies focused on the coelomic
cavity. 33
Although 18S rRNA is widely used in molecular phylogeny studies, it has limits such
as long- branch chain attractions and a slow rate of evolutionary change. Long-branch
chain attractions are a phenomenon in phylogenetic analyses when rapidly evolving
lineages are inferred to be closely related, regardless of their true evolutionary
relationships. Hence other genes are used in this study to complement the 18S rRNA
such as the cytochrome oxidase genes. The cytochrome oxidase genes are highly conserved throughout all eukaryotic organisms and they are less ambiguous to align as
compared to the ribosomal genes, making them better phylogenetic markers as compared
to the 18S rRNA gene.
This study focuses on using a molecular approach (ARDRA, PCR, phylogenetic tree
reconstruction) to determine the phylogeny of pelagic chaetognaths found on Pelican
Island, Galveston, Texas. 18S rRNA, Cytochrome Oxidase I and Cytochrome Oxidase II
genes were used to help decipher the phylogeny of this group.
All analyzed genes in this study (18S rRNA, COI, and COII) grouped the Pelican
Island chaetognaths with the protostomes. The maximum parsimony bootstrap tree for
the 18S rRNA gene, grouped the samples closest to the arthropods (protostome). For the
COI and COII genes, the minimum evolution bootstrap tree grouped the 8 collected
samples more closely to two other protostome phyla: the mollusks and annelids (COI)
while bootstrapping with the COII grouped the samples with the nematodes (with >66 percent
bootstrap). My findings are significant because they reveal phylogenetic results of a
protostome lineage for the Chaetognatha using 3 genes, one of which (COII) has not
been greatly studied for the Chaetognatha.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-2010-12-8622 |
| Date | 2010 December 1900 |
| Creators | Towers, Leah Nicole |
| Contributors | Benedik, Michael |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Thesis, text |
| Format | application/pdf |
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